Television and like system



Jan. 30, 1940. G. DovAsToN ET AL 2,188,679

TELEVISION AND LIKE SYSTEM Filed May 27, 1939 l//oEa AMPLIFIER.

PART 0F /wx/L/ARY MPL/HER carpar- ATTO RN EY Patented Jan. 30, 1940 vTELEvisIoN ANnLiKE SYSTEM George Dovaston and George Edward GordonGraham, London, England, assignors to Baird Television Limited, London,England, a British Company Application May 27, 1939, Serial No. 276,160In Great Britain November 5, 1937 6 Claims.

This invention relates to improvements in television and like system andis especially concerned with systems of the type in Which at atransmitter the luminescent screen of a cathode ray tube is scanned toproduce a moving light spot which is projected upon the object to bescanned so that light passing through, in the case of a transparentobject, or diffusely reflected from, in the case of an opaque object,becomes incident upon a light sensitive device to give rise to an imagesignal. It is found that in systems of this type uneven thickness of thefluorescent-material, relatively insensitive portions of materialforming the screen of the cathode ray tube and uneven thickness of theglass end Wall of the tube cause an uneven background in the receivedimage.

The present invention seeks to overcome this dimculty either wholly orin part by the provision of means whereby the brightness of the scanningspot is maintained substantially constant irrespective of localvariations in the activity of the fluorescent screen or of the value ofray current e. g. due to hum, interference splashes and so on.

According to the present invention in a television or like system of thetype described there is provided means for compensating for localvariations in the brightness of the scanning spot on the screen of thecathode ray tube.

According to a feature of the present invention in television or likesystem of the type referred to there is provided an auxiliary lightsensitive device which is aected only by light incident directly thereonfrom the uorescent screen of a cathode ray tube, the output from thesaid light sensitive device being employed to oontrol the intensity ofthe electron beam incident upon the said screen.

According to an alternative feature of the present invention the outputfrom the said auxiliary light sensitive device is employed to controlthe gain of an amplier through which the signal derived from thesignal-producing light sensitive device is passed.

In order that the present invention may be more particularly describedreference is now made to the accompanying diagrammatic drawing whichillustrates in Figs. l and 2 by Way of example two embodiments of theinvention.

In Fig. 1 of the drawing the luminescent screen l of a cathode ray tube2 is scanned and an image of the screen l is projected upon thetransparency 3 by means of an optical system 4. Light passing throughthe transparency 3 is received upon a light sensitive cell 5 Which feedsa vision amplifier 6. An auxiliary light sensitive cell is arranged toreceive light from any part of the luminescent screen l and the outputfrom this light sensitive cell is amplified by an aux- "5 iliaryamplifier S and is used to control the amplification `of the amplifierin such a way that when the brightness of the luminous spot on thescreen l increases, the amplification of the amplier 6 decreases andvice versa, the system 10 being adjusted so that the image signal outputfrom the amplier 6 remains substantially constant independently of thebrightness variation, Within a certain range, of the scanning spot onthe screen l. 1

Alternatively, or in addition, as shown in Fig. 2 part of the outputfrom the amplifier 8 may be used to control the intensity of theelectron beam scanning the screen l so as to bring the brightness of theluminescent spot on the screen l to some predetermined level. Theauxiliary cell 'l should be placed at some distance from the screen l sothat it is affected equally by light spots at different positions on thescreen I but if this condition cannot be fulfilled the ampliiication ofthe auxiliary amplifier 8 may be caused to vary with the position of thescanning spot on the screen I. This may be effected by means of avariable-mu valve in the amplifier 8 which is arranged to be biassed bya signal derived from the scanning generators controlling the electronbeam in the tube 2.

We claim:

1. A television system comprising a cathode ray tube having aluminescent screen exposed to an electron beam, means for projecting thescanning spot of light formed by said beam on said screen upon an objectto be scanned, a light sensitive device for receiving the light after ithas been incident upon said object, means connected to said lightsensitive device for providing an image vsignal corresponding to thelight and shade of the object, an auxiliary light sensitive devicearranged to receive light directly from said screen, and means connectedto said auxiliary light sensitive device to compensate for localvariations in the brightness of the spot of light on the screen.

2. A television system comprising a cathode ray tube having aluminescent screen exposed to an electron beam, means for projecting thescanning spot of light formed by said beam on said screen upon an objectto be scanned, a light sensitive device for receiving the light after ithas been incident upon said object, means connected M to said lightsensitive device for providing an image signal corresponding to thelight and shade of the object, an auxiliary light sensitive devicearranged to receive light directly from said screen, and means connectedto said auxiliary light sensitive device for controlling the intensityof said electron beam.

3. A television system comprisingl a cathode ray tube having aluminescent screen exposed to an electron beam, means for projecting thescanning spot of light formed by said beam on said screen upon an objectto be scanned, a light sensitive device for receiving the light after ithas been incident upon said object, means comprising an image signalamplier connected to said light sensitive device for providing an imagesignal corresponding to the light and shade of the object, an auxiliarylight sensitive device arranged to receive light directly from saidscreen, and means connected to said auxiliary light sensitive device forcontrolling kthe gain of said irnage signal amplifier.

4. A television system comprising a` cathode ray tube having aluminescent screen exposed to an electron beam, means for projecting thescanning spot of light formed by said beam on said screen upon an objectto be scanned, a light sensitive device for receiving the light after ithas been incident upon said object, means connected to said lightsensitive device for providing an image signal corresponding to thelight and shade of the object, an auxiliary light sensitive devicearranged to receive light directly from said screen, means connected tosaid auxiliary light sensitive device to compensate for local variationsin the brightness of the spot of light on the screen, and means forcontrolling said last mentioned means so that for a given brightness ofscanning spot on said screen it is substantially independent of theposition of the said spot of light on said screen.

5. A television system according to claim 1 wherein the light spot isprojected upon a transparent object so that light passing therethroughis incident upon the light sensitive device.

6. A television system according to claim 1 wherein the light spot isprojected upon an opaque object so that light diiiusely reflectedtherefrom is incident upon the light sensitive device.

GEORGE] DOVASTON. GEORGE EDWARD GORDON GRAHAM.

